The instant invention is directed to the production of spontaneous opal glasses which contain amorphous particles as the opacifying or light-diffusing phase. Thus, the glasses are phase separated, but non-crystalline, exhibit a very dense, milky-white appearance (unless colorants are added thereto), and strongly resist weathering and attack by acids and alkaline detergents, thereby recommending their utility for culinary ware and tableware.
This type of glass has been termed an immiscible opal, i.e., an opal glass wherein the opacifying phase is a glass which is insoluble in the basic glass matrix. Numerous attempts have been pursued in the past to develop an example of this type of opal glass which combines good chemical durability and resistance to weathering with a dense, white opacity.
To illustrate, U.S. Pat. No. 3,498,801 describes the preparation of opal glasses utilizing a liquid-liquid phase separation to impart opacification thereto, the glasses having compositions consisting essentially, in weight percent on the oxide basis, of 9-13.5% alkali metal oxide, the presence of Na.sub.2 O being required, 1-2%, CaO, 4-8% Al.sub.2 O.sub.3, 5-12% B.sub.2 O.sub.3, 60-68% SiO.sub.2, and 3-5.5% P.sub.2 O.sub.5. Up to 1.5% total of other alkaline earth metal oxides may optionally be included. TiO.sub.2 and F are nowhere mentioned.
U.S. Pat. No. 3,506,464 discusses immiscible opal glasses consisting essentially, in mole percent on the oxide basis, of 3-6% Na.sub.2 O, 0-6% Al.sub.2 O.sub.3, 7-11% B.sub.2 O.sub.3, 78-85% SiO.sub.2, and 1-6% P.sub.2 O.sub.5. Alkaline earth metal oxides, TiO.sub.2, and F are nowhere referred to in the specification.
U.S. Pat. No. 3,661,601 discloses immiscible opal glasses consisting essentially, in weight percent on the oxide basis, of 3-10% Na.sub.2 O+K.sub.2 O, 11-20% CaO, 1-7% B.sub.2 O.sub.3, 3-9% Al.sub.2 O.sub.3, 50-75% SiO.sub.2, and 2-4% F. Up to 10% total of BaO, SrO, P.sub.2 O.sub.5, La.sub.2 O.sub.3, TiO.sub.2, Nb.sub.2 O.sub.5, ZnO, GeO.sub.2, PbO, Bi.sub.2 O.sub.3, and CeO.sub.2 may be present. The opacifying droplets are believed to contain CaO and F or a combination of CaO, F, B.sub.2 O.sub.3, and SiO.sub.2.
U.S. Pat. No. 3,723,144 is concerned with immiscible opal glasses consisting essentially, in weight percent on the oxide basis, of 0.5-2.5% Li O, 7-10% ZnO, 11-14% B.sub.2 O.sub.3, and 71-76% SiO.sub.2. Up to 5% total of Na.sub.2 O+K.sub.2 O+Al.sub.2 O.sub.3 may be present in the indicated proportions of 0-0.5% Na.sub.2 O, 0-3% K.sub.2 O, and 0-1.5% Al.sub.2 O.sub.3. The alkaline earth metals, TiO.sub.2, and F form no part of the compositions.
U.S. Pat. No. 3,728,139 relates to the manufacture of phase separated opal glasses consisting essentially, in weight percent on the oxide basis, of 0-10% Li.sub.2 O and/or Na.sub.2 O and/or K.sub.2 O, 8-15% B.sub.2 O.sub.3, 0-15% of bivalent metal oxides selected from the group of BaO, CaO, CoO, CuO, MgO, MnO, NiO, and ZnO, 0-5% TiO.sub.2 and/or ZrO.sub.2, 0-3% Al.sub.2 O.sub.3, 70-80% SiO.sub.2, and 1-3% As.sub.2 O.sub.3 and/or MoO.sub.3 and/or WO.sub.3. F is nowhere mentioned.
Whereas each of those disclosures describes opal glasses demonstrating relatively dense opacity and good chemical durability and resistance to weathering, the requirements for materials to be used for culinary and table ware have become more and more stringent, both with respect to the aesthetic appearance and the chemical durability of the ware. Accordingly, research has been constant to develop even better materials.